Sound absorption
16 November 2012
Sound Absorption
1
Sound against a wall
• Balance of sound energy
impinging over a wall
• The energy balance shows
three main fluxes:
– Reflected
– Absorbed
– Transmitted
• Hence three coefficients are
defined, as the ratios with the
impinging energy
16 November 2012
Sound Absorption
r+a+t =1
2
Materials: sound insulating & sound absorbing
Sound absorbing materials must not be confused with sound insulating
materials:
Sound Insulating material:
Heavy and stiff, minimizes
transmitted power “Wt”.
the
Sound Absorbing material:
Soft and porous, minimizes
reflected power “Wr”.
16 November 2012
the
Sound Absorption
3
Sound absorption = weak reflection
• If the surface is large compared to wavelength, the
reflection happens specularly, as a light ray (Snell’s
Law).
S
S’
16 November 2012
Sound Absorption
4
Sound absorption = weak reflection
Regarding sound absorption
IT DOES NOT MATTER
if sound energy is dying inside the wall or is passing through
16 November 2012
Sound Absorption
5
Sound absorbing materials: generalities (1)
When the the noise source is in the same room as the receiver, the noise level
can be reduced in three ways:
• reducing the sound power radiated by the source,
• bringing the receiver far away from the source ( r < rc),
• reducinhg the reflected energy (r > rc).
The latest effect is obtained by increasing the equivalent absorption area A,
which is given by:
• A =  i Si
( m2 )
where Si and i are respectively area and absorption coefficient of the i-th
surface surrounding the room.
16 November 2012
Sound Absorption
6
Sound absorbing materials: generalities (2)
Inside the purely reverberant sound field (r >> rc), the sound level reduction
DL caused by the increase of sound absorption is given by:
• DL (f) = 10 log (A2/ A1)
(dB)
where 1 and 2 refer to the values before and after the introduction of the
absorbers.
Sound absorbing materials are usually classified in the following categories:
a) porous materials,
b) acoustic resonators,
c) vibrating panels,
d) hybrid systems.
16 November 2012
Sound Absorption
7
SOUND ABSORBING MATERIALS
Porous materials
Suspended ceiling, hanging
under the structure
Foam attached directly to the
structure
16 November 2012
Sound Absorption
8
SOUND ABSORBING MATERIALS
Absorption coefficient vs. frequency & thickness
Increasing the thickness of a porous layer attached directly on a rigid wall
improves absorption at low frequency
16 November 2012
Sound Absorption
9
SOUND ABSORBING MATERIALS
Effect of distance from a rigid wall
16 November 2012
Sound Absorption
10
SOUND ABSORBING MATERIALS
Acoustic
resonators
f0 
c0
2
r
2
Scheme of an Helmoltz’s resonator
 

V l 

2r 

c0 : sound speed (m/s)
r : neck’s radius (m)
l : neck’s length (m)
V : volume of rear cavity (m3)
16 November 2012
Sound Absorption
11
Effect of damping material inside
SOUND ABSORBING MATERIALS
Vibrating panels
Scheme of a vibrating panel
f0 =
60
(s × d )
16 November 2012
 : surface density of panel (kg/m2)
d : distance panel – wall (m)
Sound Absorption
12
SOUND ABSORBING MATERIALS
Hybrid systems
Schemes of hybrid systems
16 November 2012
Sound Absorption
13
SOUND ABSORBING MATERIALS
Vibrating panels
Hybrid systems
Resonators
16 November 2012
Sound Absorption
14
Example - Casa della Musica
Helmoltz Resonators
Porous panels
Vibrating panels
“Bass Trap”
16 November 2012
Sound Absorption
15
Sound Scattering Coefficient s
Wdiff
s=
Wdiff +Wspec
• On a rough surface, a fraction s of the reflected energy will
be radiated diffusely, while the remaining fraction 1-s will be
radiated specularly
Specular component
Wspec =Winc·(1-α) · (1s)
Diffused component
Wdif =Winc·(1-α) · s
16 November 2012
Sound Absorption
16
Acoustic diffusors
• Most diffusing panels are done by:
• pseudo-random cavities
• 2D and 3D scattering versions
• Curved surfaces
Reference brand: RPG Diffusor Systems
(http://www.rpginc.com)
16 November 2012
Sound Absorption
17
Acoustic diffusors
Curved panels are often used for building acoustical shells in
theatres
Reference brand: Wenger
(http://www.wengercorp.com)
16 November 2012
Sound Absorption
18